Alloy and finishing system



Sept. 3, 1968 H. J. COATES ET AL 3,400,057

ALLOY AND FINISHING SYSTEM Filed June 5, 1964 INVENTORs fi e/aid Jiyfersarz fades 36272;? 124? %rd, J21

ATTORNEXj United States Patent 3,400,057 ALLOY AND FINISHING SYSTEM Harold Jefferson Coates and Bennie Ray Ward, Jr., Henrico County, Va., assignors to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Filed June 3, 1964, Ser. No. 372,200 9 Claims. (Cl. 204--33) ABSTRACT OF THE DISCLOSURE A panel of high silicon aluminum alloy suitable for architectural panelling and for blackboard and writing slates, and having a sparkling appearance, is prepared by homogenizing a high silicon aluminum alloy ingot in the as-cast condition at about 1000 to 1100 F., hot rolling to form a plate, preferentially etching with alkali to expose particles of silicon protruding above the surrounding alloy surface, thereby enriching the surface in silicon, and then anodizing to produce an anodic coating having exposed silicon particles embedded therein.

This invention relates to a novel method for imparting to high silicon aluminum base alloys an anodized finish having a sparkling appearance, and to the finished alloys thus produced. More particularly, the invention concerns a novel blackboard or other writing surface for chalk or crayon comprising a plate of said finished aluminum alloy.

Modern architectural design has increasingly employed anodized aluminum paneling and other elements to obtain novel and striking esthetic effects. Such effects have, however, been largely confined either to matte or to lustrous finishes.

In accordance with the present invention, there is provided a novel method whereby there is produced on the surface of high silicon aluminum base alloys an anodized finish which possesses an attractive sparkling appearance when viewed in strong directional incandescent light or in sunlight. The finish ranges in color from dark gray to greenish or olive hues. This finish renders the aluminum alloys thus treated especially suitable for the production of architectural paneling. It also provides the alloy with a type of surface which is well suited for receiving markings of chalk, crayon, and other writing materials, and hence the treated alloy of the invention is particularly adapted for use as a blackboard and writing slate, in schools and for similar purposes.

The expression high silicon aluminum base alloys as employed herein refers to aluminum base alloys containing from about to about 9% silicon by weight, as well as to such alloys which further contain small amounts of chromium, ranging from about 0.4% to about 0.7%, or small amounts of iron.

The high silicon alloys are suitably obtained in the form of cast ingots. The novel treatment of the invention for imparting to the surface of the alloys the aforementioned sparkling effect, comprises the steps of (1) homogenizing the alloy in the as-cast condition by long heating at a temperature below the eutectic melting temperature; (2) hot rolling the homogenized alloy to obtain a plate of desired thickness; (3) preferentially removing aluminum at the surface of the alloy to expose particles of silicon protruding above the surrounding aluminum alloy surface, thereby enriching the surface in silicon, and (4) anodizing the treated surface to produce thereon an anodic coating having silicon particles randomly oriented therein.

The resulting surfaces are especially adapted for use as schoolroom blackboards. The use of anodized aluminum to provide a writing surface has already been proposed, and is disclosed, for example, in US. Patent 2,202,682,

3,400,057 Patented Sept. 3, 1968 but such simple anodized coatings have a smooth, satiny quality, and do not posses the unusual sparkling effect provided by the present invention.

The first, or homogenizing step, of the process is carried out, in accordance with the invention, by heating an ingot of high silicon aluminum base alloy in the as-cast condition in a suitable furnace, for a period of from about 50 to 70 hours at a temperature below the eutectic melting temperature of the alloy. This temperature will generally be in the range of 1000 F. to 1100 F. Thus, for example, an alloy containing about 8% silicon will typically be heated at about 1070 F. for 62 hours, while an alloy containing 5% silicon and 0.5% chromium will be homogenized for 64 hours at 1050 F. The homogenizing step produces material which when fabricated, etched, and anodized, will yield finishes with large silicon particles exposed, which provide the desired sparkle effect.

Following the homogenizing treatment, the ingot is scalped and preheated at elevated temperature, for example, about 800 F. to about 900 F. for 1 to 3 hours, and then hot rolled to any desired plate thickness, for example 0.5 to 0.7 inch. It is then cooled to room temperature, and can be further cold rolled to any desired final thickness, for example, from about 0.02 to 0.04 inch. The cold rolled sheet may, if desired, be given a final annealing treatment, for example, by heating at about 800 F. for 30 minutes.

In accordance with the invention, the preferential removal of aluminum at the surface of the alloy sheet to expose particles of silicon protruding above the surface, thereby enriching the surface in silicon, is advantageously performed by a chemical etching treatment. There are employed for this purpose, conventional baths of the type used in etching and bright dipping of aluminum alloys. In accordance with a preferred practice of the invention, the sheets to be treated are given a brief preetch in an acid bath, such as a solution of phosphoric acid and nitric acid, or of nitric acid and hydrofluoric acid. Thus, for example, the sheet is given a 15 second preetch in a bath of 3% HF and 10% HNO by weight, to insure uniformity of subsequent etching in an alkaline bath.

For the removal of aluminum there is advantageously employed an alkaline type treating bath, such as a solution of sodium hydroxide or sodium carbonate, which includes a small amount of sodium gluconate. Thus, there may be used a solution of sodium hydroxide containing from about 3 to 9 ounces (2% to 6%) of solid sodium hydroxide per gallon of solution, the preferred concentration being about 6 ounces (5%) NaOH, or 45 grams per liter. Sodium gluconate is present in about 0.01 lb. per gallon. The treatment temperature range is from about F. to 160 F., preferably about F., While the time of treatment is from 2 to 10 minutes, preferably 5 minutes.

Following the etching step, the sheets may be given a desmutting treatment by dipping for about 30 seconds in an acid bath comprising a solution of 3% hydrofluoric acid and 10% nitric acid, followed by rinsing with Water.

The etched aluminum alloy sheets are then subjected to the anodizing step, which is performed using conventional baths and anodizing conditions. Thus, the anodizing bath is advantageously a solution of from about 15% to about 20% sulfuric acid, and preferably 17% strength. The anodizing temperature is 70 F. Current density ranges from about 10 to 20 amperes per sq. ft. Anodizing time is from to 1 /2 hours, preferably one hour. The voltage (DC) is initially about 17.5 volts, rising to about 22 volts.

The high silicon aluminum base alloys thus anodized yield a gray to dark gray coating. Those containing chromium yield a dark gray coating with a slight green or olive background. Both possess an unusual sparkling appearance. The sparkling effect is believed to be due to the size and shape of the exposed silicon particles which are embedded in the surface of the anodic coating.

The treated aluminum alloy sheets, prepared as described, exhibit qualities of ease of writing with chalk and crayon, erasability, total reflectance, and abrasion resistance.

The preparation of a high silicon aluminum base alloy sheet suitable for blackboard use is illustrated by the following example, which is not, however, to be regarded as limiting.

Example 1 A 20 lb. cast ingot of aluminum alloy containing silicon and 0.5% chromium was homogenized by heating for 64 hours at 1050 F. The ingot was then hot rolled from 2.5 inches to 0.625 inch thickness, reheated to 800 F. for /2 hour and then hot rolled to 0.140 inch thickness. It was cooled to room temperature and cold rolled to 0.034 and 0.023 inch. The cold rolled sheet was given a final annealing treatment at 800 F. for 30 minutes.

The sheet 12" x 24" in area was given a 15 second preetch in 3% HF, HNO solution to insure uniformity of etching, and then etched in a 5% NaOH solution for 5 minutes at 150 F. The sheet was desmutted by means of a 30 second dip in a 3% HF-10% HNO bath, rinsed, and mounted on an anodizing rack. -It was anodized in 17% H 50 solution for 1 hour at 70 F. at a current density of amps. per sq. ft., with an initial DC. voltage of 17.5, and a terminal voltage of 22 volts. A dark gray coating with a slight green background was obtained, which possessed a sparkling appearance.

The applicability of the invention to the manufacture of blackboards and similar writing surface structures may be better understood by reference to the accompanying drawing, in which:

FIGURE 1 is a plan view of an aluminum alloy blackboard embodying the invention;

FIGURE 2 is a cross-sectional view of a blackboard element showing the treated surface thereof.

Referring to FIGURES 1 and 2, a blackboard element 10 comprising a high silicon aluminum base alloy is mounted in a frame 11. The backboard element 10 comprises a plate or sheet 12 of the treated aluminum alloy, mounted on a suitable backing material 13, such as, for example, plywood, by a suitable adhesive, such as an epoxy resin.

The aluminum alloy surface is provided with an anodized coating 14 in which there are embedded particles of silicon which extend toward and protrude beyond the surface, forming a jagged, light reflecting surface, shown generally at 15, which imparts to the surface a grain rendering it receptive to chalk or crayon markings, which are readily erasable by means of felt or similar erasers.

What is claimed is:

1. Method of imparting to the surface of an aluminum base alloy containing from about 5% to about 9% silicon by weight an anodized finish having a sparkling appearance, comprising the steps of:

(a) homogenizing an ingot of the alloy in the as-cast condition by heating for a period of about 50 to 70 hours at a temperature between about 1000 F. and about 1100 F.;

(b) hot rolling the alloy to form a plate;

(0) preferentially removing aluminum from a surface layer of said plate by etching with an alkaline bath to expose particles of silicon protruding above the surrounding alloy, thereby enriching the surface in silicon; and

(d) anodizing the treated alloy surface to produce thereon an anodic coating having silicon particles randomly embedded therein and extending toward and protruding beyond thesurface and forming a jagged, light reflecting and grained. surface.

2. The method of claim 1 in which the alloy contains further from about 0.4% to about 0.7 chromium.

3. The method of claim 1 in which the alkaline bath is an approximately 5% sodium hydroxide solution.

4. The method of claim 1 in which the anodizing step is performed in sulfuric acid bath at a current density of about 10 to about 20 amperes per sq. ft., and a direct current voltage from about 17.5 volts to about 22 volts.

5. In the method of imparting to the surface of an aluminum base alloy containing from about 5% to about 9% by weight of silicon an anodized finish having a sparkling appearance, and including the steps of homogenizing an ingot of the alloy in the as-cast condition and hot rolling to form a plate, the improvement which comprises:

(a) preferentially removing aluminum from a surface layer of said plate by etching with an alkaline bath to expose particles of silicon protruding above the surrounding alloy; and

(b) anodizing the treated alloy surface to produce thereon an anodic coating having silicon particles randomly embedded therein and extending toward and protruding beyond the surface and forming a jagged, light reflecting and grained surface.

6. An anodized article of an aluminum base alloy containing from about 5% to about 9% by weight of silicon having on its surface a finish comprising an anodic coating having silicon particles randomly embedded therein, and extending toward and protruding beyond the surface and forming a jagged, light reflecting and grained surface having a sparkling appearance.

7. The article of claim 6 in which the alloy contains further from about 0.4% to about 0.7 chromium.

8. A blackboard writing surface comprising a plate of an aluminum base alloy containing from about 5% to about 9% by weight of silicon having on its surface a finish comprising an anodic coating having silicon par ticles randomly embedded therein, and extending toward and protruding beyond the surface and forming a jagged, light reflecting and grained surface receptive to chalk and crayon markings. I

9. The blackboard of claim 8 in which the alloy contains further from about 0.4% to about 0.7% chromium.

References Cited UNITED STATES PATENTS 2,202,682 5/1940 Arberry 35-66 2,262,696 11/1941 Nock et al. 148-4 2,040,617 5/1936 Mason et al. 204-1 2,851,626 9/1958 Allard et al. 313-70 3,041,259 6/1962 Stoddard 204-141 3,020,219- 2/ 1962 Franklin et al 204-58 3,153,278 10/1964 Martin et al. 29-424 3,164,494 1/1965 English 148-315 3,180,806 4/ 1965 Hollingsworth 204-29 3,266,945 8/1966 Helling et al. 148-115 3,328,274 6/ 1967 Bushey et a1 204-58 HOWARD S. WILLIAMS, Primary Examiner. W. VAN SISE, Assistant Examiner. 

